This invention relates to a timepiece, and more particularly to compensating for delays in the second hand movement of a timepiece following time correction activities.
When correcting the time of a step driven or sweep driven electronic watch, a second hand of the watch is prevented from moving by pulling a winding stem of the watch outwardly. Generation of motor driving pulses is also halted. Typically, the second hand is stopped at its zero second position (i.e. with the second hand over the numeral 12 on the face of the watch) based on generally accurate information provided by a radio, television or the like. Following time correction, the winding stem is pushed inwardly. Approximately one second is required after the winding stem has been pushed inwardly before the first of the motor driving pulses is once again generated.
A second hand readjustment type device for readjusting a driving shaft and a driven shaft of a sweep driven watch employing a magnetic coupling mechanism and viscous member is disclosed in Japanese Patent Laid-Open No. 87066/75. Another device for readjusting a wheel train on a driven shaft side of a watch is disclosed in Japanese Patent Laid-Open No. 161581/87. The first of the motor driving pulses produced after time correction activities have been completed in these devices occurs approximately one second later assuming a motor driving frequency of 1 Hz and 1/N second later when the motor driving frequency is N Hz.
Conventional sweep driven watches require the winding up of an energy storage member such as a hairspring or the like by a step motor. A balancing of forces between a recoil torque produced by the hairspring and a load torque generated by member (e.g. a rotor) within a viscous fluid (e.g. oil) provides smooth second hand movement. More particularly, the oil rotor generates a load torque proportional to angular velocity which resists changes in the recoil torque produced by the hairspring. The load torque increases as the recoil torque increases and decreases as the recoil torque decreases. The angular velocity of the hairspring as it recoils is relatively constant.
The hairspring is connected to the second hand through a wheel/gear train. A smooth sweeping motion of the second hand results. To maintain a uniform sweeping motion of the second hand, the recoil torque of the hairspring is reinforced by a predetermined periodic driving force produced by a step motor supplied to the hairspring. The recoil torque of the hairspring and the load torque of the oil rotor are generally balanced at all times.
An imbalance between the recoil torque of the hairspring and the load torque of the oil rotor, however, occurs immediately following time correction of the watch. In particular, the balance between these two torques fluctuates due to the periodicity of energy fed to the hairspring from the step motor being disturbed. A change in the angular velocity of the second hand results. Once the imbalance passes, that is, once energy is again fed on a uniform periodic basis to the hairspring the second hand will resume rotating at a constant angular velocity. Generally, during the imbalance between the recoil torque and load torque, the variation in the angular velocity of the second hand movement causes the second hand to move at a slower than real time rate. Therefore, the time displayed by the watch is incorrect. The time inaccuracy is particularly conspicuous at the time of correction.
For example, assume the winding stem of a watch is pulled out 0.8 second after the generation of a driving pulse by a step motor having a driving frequency of 1 Hz. The wheel train connected to the drive shaft of the hairspring is readjusted to correct the displayed time. As long as the winding stem is pulled out, the stored energy within the hairspring remains constant. Once the winding stem is pushed in following time correction of the watch, the hairspring gradually recoils. Approximately one second after the winding stem has been pushed in, a driving pulse from the motor is generated. Sweep movement of the second hand results. The hairspring has received driving pulses 0.8 second prior to the winding stem being pulled out and one second after the winding stem is pushed in, that is, an interval of 1.8 second between driving pulses. To maintain a balance between the recoil torque and the load torque, however, the driving pulses need to be provided to the hairspring every 1.0 seconds. The additional 0.8 seconds during which no driving pulse is provided to the hairspring results in a torque imbalance. The angular velocity of the second hand temporarily decreases resulting in the time displayed by the watch being incorrect. Once balance is restored between the recoil torque and load torque, the second hand resumes its smooth movement. Nevertheless, the time displayed by the watch is now incorrect.
When the motor driving frequency is 1 Hz, the time delay will be no greater than 1 second. Sweep driven watches using the foregoing coupling mechanism are not limited to motor driving frequencies of 1 Hz and can arbitrarily choose other motor driving frequencies which can result in greater time delays. These time delays are also due, in part, to the reduction ratio of the wheel train employed within the watch. Generally, when the motor frequency is N Hz, the time delay is no greater than 1/N seconds. The lower the motor frequency, the more conspicuous the time delay becomes.
It is therefore desirable to provide a timepiece having a second hand sweep-driven movement in which accurate time correction can be made and maintained by balancing the recoil torque of the hairspring and the load torque of the oil rotor at all times and especially immediately after time correction activities have been completed.